Nowadays, compound semiconductors such as gallium-arsenic compounds, gallium-phosphorus compounds, etc. are widely used for various purposes, and the demand therefor is rapidly increasing. Semiconductor chips of GaAs, GaP, etc. are prepared by forming monocrystals of GaAs, GaP, etc. by a drawing-up method, slicing the formed monocrystal rod into wafers, and cutting the wafers into chips of several millimeters square. In these procedures, a large amount of scrap is produced during the several steps of slicing and cutting. Also, when monocrystals are prepared, irregularly shaped portions formed at the start of the drawing-up are discarded, and a considerable amount of polycrstalline materials remain at the bottom of the crucibles used for melting. All these from gallium-containing waste.
Gallium is a rare element having no specific ores. Therefore, it is essential to recover gallium from the above-mentioned waste and reuse it. Various methods for recovering gallium have been known.
Japanese Patent Publication No. 56-386641B(1981) discloses a process, which comprises dissolving waste containing gallium and arsenic in an acid medium in the presence of an oxidizing reagent, adjusting the pH of the solution to 2-8 to let hydroxides of gallium and arsenic precipitate, collecting the precipitate by filtration, dissolving the precipitate in an alkaline solution as sodium gallate, and finally recovering gallium electrolytically.
Another example is a process of Canadian Patent No. 1094328, which comprises dissolving waste containing gallium and arsenic in an acid, removing the residue by filtration, raising the pH of the solution to 11 or higher with NaOH, precipitating arsenic as calcium arsenate by addition of CaO or Ca(OH).sub.2, separating the precipitate by filtration and electrolytically recovering gallium.
The above-mentioned wet processes generally comprise dissolving gallium-containing waste in an acid or alkaline aqueous medium, separating unwanted and wanted materials as a precipitate and a filtrate and finally recovering gallium electrolytically from the filtrate. However, the gallium recovered by the above processes is not satisfactory in purity as semiconductor material and requires a further purification. Also, wet processes generally consume much time and energy.
Another example is a process of Japanese Patent Publication 57-101625A(1982), wherein gallium-containing waste is subjected to vacuum distillation at a high temperature of 1100.degree.-1150.degree. C., the sublimed arsenic is recovered by condensation, and the molten gallium is cooled, dissolved in an acid and recovered by recrystallization in a purified state. This process requires high-temperature distallation which consumes a lot of energy and is not satisfactory in that when chromium is present in the waste, high quality gallium cannot be recovered because separation of chromium and gallium is difficult by this process.
Rather recently, a chlorination process has been proposed in chlorine, arsenic chloride is separated by distillation and gallium chloride is recovered (U.S. Pat. No. 4,666,575). This chlorination process is advantageous in that gallium of higher purity can be recovered more easily than by previously known processes. However, this chlorination process is not applicable to gallium waste containing no arsenic, since this chlorination process is characterized by performing chlorination of gallium perventing precipitation of gallium dichloride, which causes blocking of the chlorine-blowing lance, in the presence of arsenic chloride. Therefore, when gallium waste containing no arsenic is treated, metallic arsenic or arsenic chloride must be added. The chlorination must be performed at a temperature lower than the boiling point of arsenic chloride (Ca. 130.degree. ), preferably under 70.degree. C., until substantially all of the gallium in the waste is chlorinated inspite that the arsenic chloride has finally to be distilled off.
We attempt improvement of the above-mentioned chlorination process and have found that chlorination of gallium-containing waste effectively proceeds if the chlorination thereof is carried out in the presence of gallium trichloride from the beginning, chlorination can be performed at a higher temperature, and chlorination and distillation of arsenic chloride and other low-boiling-point chlorides can be simultaneously conducted.